Anterior cervical plating system and bone screw

ABSTRACT

Anatomically contoured anterior cervical plates with bone ingrowth surfaces, providing for intersegmental compressive preloading, and a rigid and locked interface to all of the bone screws, with those engaging the vertebrae deployed in highly convergent pairs. The bone screws have a tapered self-tapping leading end, an increasing root diameter with a generally constant outer diameter with a thread that is narrow and sharp throughout and an enlarged head portion capable of an interference fit to the receiving holes of the plate. Instrumentation consists of plate holders, a compression apparatus and a pilot hole forming device that interlocks with the plate. Methods for spinal compression and bone hole preparation are provided.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/618,036, nowU.S. Pat. No. 6,620,163 filed Jul. 17, 2000; which is a divisional ofapplication Ser. No. 09/022,293, now U.S. Pat. No. 6,193,721 filed Feb.11, 1998; which claims benefit of U.S. provisional application Ser. No.60/037,139, filed Feb. 11, 1997; all of which are incorporated herein byreference. application Ser. No. 09/022,344, filed Feb. 11, 1998, andtitled SKELETAL PLATING SYSTEM, now U.S. Pat. No. 6,139,550, isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to implants, method, andinstrumentation for fusion of the human cervical spine from the anterioraspect, and in particular to plate systems for aligning and maintainingadjacent cervical vertebrae in a selected spatial relationship duringspinal fusion of those vertebrae.

2. Description of the Related Art

It is current practice in the art to use cervical plating systems forthis purpose. Such systems are composed essentially of plates and screwsfor aligning and holding vertebrae in a desired position relative to oneanother. The earliest such devices consisted of stainless steel platesand screws and required that the screws passed entirely through thevertebrae and into the spinal canal in order to engage the strong bonetissue (the posterior cortex) of the vertebral bodies. This required theability to observe or visualize this area radiographically, which is notalways possible, especially in the lower cervical spine where thevertebrae may be hidden radiographically by the shoulders.

In order to form holes in the vertebral bodies for insertion of eachscrew, a drilling operation was performed, followed by a tappingoperation. Each of these operations involved the passage of aninstrument entirely through the associated vertebral body and into thespinal column. Thus, these instruments come into close proximity to thespinal cord and the dural sac which are in close proximity to the backsurfaces of the vertebral bodies. Any procedure which introduces anobject into the spinal canal presents serious risks which are of concernto the surgeon.

The conventional technique of forming a bone screw receiving hole invertebral bodies by drilling has a number of significant disadvantages.For example, drilling removes bone material, leaving a void andresulting in a loss of bone material. Drilling also causesmicrofracturing of the bone at the drill bit-bone interface and theresulting fracture lines tend to propagate in directions perpendicularto the wall of the hole. More specifically, the bone material isessentially a type of ceramic which exhibits a brittle pattern offracture formation and propagation in response to drilling. Furthermore,drilling generates heat which can result in thermal necrosis of the bonematerial precisely at the interface between the bone and a subsequentlyinstalled screw, where necrosis is most harmful. Any bone which doesexperience necrosis will subsequently be resorbed by the body as part ofthe bone repair process and this can lead to the loosening of the screw.

Another problem with drilling is that the path of the drill bit isdifficult to control and since the drill bit operates by rotation, itcan wind up soft tissue about the associated plate. In addition, unlessgreat care is taken, the drill bit may be driven significantly past theposterior cortex and cause irreparable harm within the spinal canal.Finally, a drill bit may bind and fracture within the vertebral body andcan then cause serious injury as the still rotating portion of the drillbit passes into the wound, while the portion of the bit which has brokenoff may either protrude dangerously from the vertebral body or may bebroken off flush with the upper surface of the body so as to beirretrievably embedded therein. In any event, the steps that must betaken to retrieve the broken-off portion of a drill bit will inevitablyprolong and complicate the surgical procedure.

In known plating systems, there have been problems with loosening andfailure of the hardware, breakage of the screws and plates, and backingout of screws into the patient's throat area. These occurrencesgenerally require further surgical procedures to replace the brokenparts or the plates and screws entirely, and to repair any damage thatmay have been caused.

Other problems which have been encountered with known systems resultfrom the failure of the screws to achieve a sufficient purchase in thebone and the stripping of the screws. Also, the use of the known platingsystems may result in a loss of lordosis, which is the normal curve ofthe cervical spine when viewed from the side.

Known plating systems additionally experience problems in connectionwith those procedures where bone grafts are placed between vertebralbodies to achieve an interbody fusion which heals by a process called“creeping substitution”. In this process, bone at the interface betweenthe graft and a vertebra is removed by a biological process whichinvolves the production of powerful acids and enzymes, as a prelude toinvasion of the interface by living tissue and the deposition, orgrowth, of new bone. While the plates allow for proper alignment of thevertebrae and their rigid fixation, they can therefore, at the same timeunfortunately, hold the vertebrae apart while the resorption phase ofthe creeping substitution process forms gaps in the bone at the fusionsite with the result that the desired fusion does not occur. Suchfailure is known as pseudoarthrosis. When such a failure occurs, thehardware itself will usually break or become loosened from the spine,thus requiring a further surgical procedure to remove the brokencomponents and another surgical procedure to again attempt fusion.

In response to the problems described above, a second generation ofplating systems has been developed and/or proposed. These include asystem disclosed in U.S. Pat. Nos. 5,364,399 to Lowery and 5,423,826 toMorscher, as well as cervical spine locking plating systems offered bySYNTHES Spine, the DANEK ORION plate, the CODMAN SHURTLEFF plate, andthe SMITH NEPHEW RICHARDS plate, among others. The systems' formingmembers of this second generation have a number of common properties.They are all made of either a titanium alloy or pure titanium ratherthan stainless steel, to minimize adverse tissue reactions and are MRIcompatible, which stainless steel is not. The screws and the plates havebeen given increased thickness in order to achieve increased strength.The screws have larger diameters to improve their purchase withoutrequiring that they engage the posterior cortex of the vertebral bodies.Some mild longitudinal contouring of the plates is employed to allow forsome lordosis, and/or limited transverse contouring to better follow thegenerally curved aspect of the front of the vertebral bodies. Mechanismsare employed for securing the vertebral bone screws to their associatedplates in a manner to prevent the screws from backing out. While thissecond generation of plating systems represents a significantimprovement over earlier systems, certain existing problems persist,while new problems have been created.

For example, since the screws no longer extend into the posteriorcortex, it is common for the threads in the tapped screw hole to becomestripped and for the screws to fail to gain a suitable purchase. Inaddition, screw breakage continues to be experienced and occurs mostcommonly at the junction of the screw to the posterior aspect of theplate. The screws employed in both the SYNTHES system and the SMITHNEPHEW RICHARDS system are particularly vulnerable to this problembecause those screws are hollow at the level where they attach to theplate to permit the internal reception of locking screws.

In an attempt to prevent screw to plate junction breakage of the screw,more recent designs of screws have an increasing root diameter from tipto head, which thus far has resulted in a near useless stubby and bluntthread near the screw head with little holding power and little tactilefeedback to the surgeon to signal the completion of tightening prior tostripping of the screw within the bone. Based on empiric studies testingthese prior art screws, the use of a pretapped hole, rather than aself-tapping screw, was found to be preferred for pullout strength andthus these screws have not been self-tapping and thus the screw holesmust be pre-tapped. Since the thread cutting portion of a tap isnecessarily sharp and rotated to work, there is a serious risk of damageto the surrounding soft tissues when it is used. This is compounded bythe fact that the plates employed in these systems do not providesufficient long axis contouring to make full allowance for lordosis anddo not have sufficient transverse contouring to prevent rocking of theplate about its longitudinal axis and to conform to the anterior shapeof the vertebral bodies, so that these plates do not prevent soft tissuefrom creeping in from the sides and beneath the screw holes thusexposing these tissues to damage by the drill and the tap. While it ispossible, at the time of surgery, to make some change in the contouringof these plates, this is generally limited to contouring of thelongitudinal axis and quite often causes distortion of the plate's bonescrew holes and screw hole to plate junctions in a manner which has anadverse effect on the screw-plate interlock. Lack of proper contouringprevents these plates from having an optimally low profile relative tothe spine.

In some of the second generation cervical plating systems, screw backoutcontinues to occur, because these plates could not be designed to allowfor the locking of all of the screws. Specifically, while the designersof these plates recognized the importance of securing the bone screws tothe plates, they were unable to lock all of the screws and had to settlefor leaving some of the screws unlocked.

Furthermore, several of these second generation systems utilize tiny anddelicate “watchmaker” parts to achieve interlocking. These parts arecharacterized by the need to engage them with particularly delicatesmall ended screw drivers. These interlocking components are easilyrendered ineffective by any effort to alter the contours of a plateduring surgery.

Despite the improvement of these second generation plating systems overthe first problems, the problems still persist, the most important ofwhich is pseudoarthroses, and particularly “distractionpseudoarthroses”. Although these second generation plates have clearlyled to an increase in fusion rate, when a failure to produce fusionoccurs, it is generally accompanied by bone resorption along a line atthe graft-to-vertebra junction, which can be seen on a radiograph.

In the case of the weak first generation plates and screws, the platesmight hold the vertebrae apart, preventing fusion, but only until thehardware would break, relieving the distraction, and then allowing thefusion to occur. The second generation systems of plates are too strongto allow this to occur, thus requiring further surgical procedures forthe correction of the pseudoarthroses.

Compression plates are well known and are widely used in orthopedicsurgery for the stabilization of tubular bones, and sometimes also flatbones. Such plates may rely on some external compression means or may beself-compressing, relying on the ability of the screw head to slidewithin a ramped slot such that the tightening of the bone screws throughthe plate imparts a linear motion perpendicular to the screw axes. U.S.Pat. No. 5,180,381 discloses an attempt to employ such a mechanism inconnection with anterior spinal fixation.

However, it has been found that all of the proposed self-compressingplating systems have in common the need for a screw to engage both aproximal and a distal cortex, (bone casing of very dense bone material),so as to anchor the screw tip in a manner to allow the plate to moverelative to the screw when tightened rather than allowing the plate todrag the screw off axis. However, as already discussed earlier herein,when a screw is to engage the posterior cortex of the vertebral body, itis necessary for the drill and the tap which form the screw hole, aswell as the screw tip itself, to all enter the spinal canal, therebyexposing the spinal cord to damage.

While the system disclosed in U.S. Pat. No. 5,180,381 avoids such dangerby engaging the vertebral body end plate instead of the posteriorvertebral body cortex, the path of the screw is of necessity quiteshort, so that there is very little opportunity for the screw threads toachieve additional purchase within the vertebral body. It wouldtherefore appear that to the extent that the device disclosed in U.S.Pat. No. 5,180,380 is able to achieve its stated objectives, it wouldpull the front of the spine together more than the back and would notappear to compress the back of the vertebral bodies at all, thusproducing an undesirable iatrogenic loss of the normal cervicallordosis. Such a situation is disruptive to the normal biomechanics ofthe cervical spine and potentially quite harmful.

The creation of compression between adjacent vertebrae would offer anumber of advantages, including reduced distraction pseudoarthrosis,increased surface area of contact between the graft and vertebrae asslightly incongruent surfaces are forced together, increased osteogenicstimulation, since compressive loads stimulate bone formation, andincreased fusion graft and spinal segment stability.

Among the new problems created by these second generation systems is atendency for the small “watchmaker” parts used to lock the bone screwsto the plate to fall off of the driver used for attaching those parts,or out of the associated plates and to become lost in the wound. Inaddition, these small parts are quite fragile and require specializedadditional instruments for their insertion and/or manipulation.Furthermore, incorrect bone screw placement relative to the axis of aplate hole may render the screw locking mechanism unworkable or maycause sharp and jagged shavings of titanium to be formed as a lockingscrew is driven into contact with an improperly seated bone screw. Themeans for establishing bone screw to plate hole alignment andpreparation are less than reliable. Furthermore, most of these secondgeneration systems lack a reliable and effective means for positioningand holding the plate during attachment.

Specific features of various prior art systems will be summarized below.

The system disclosed in U.S. Pat. Nos. 5,364,399 and 5,423,826, citedearlier herein, includes a thin stainless steel plate which allows forside-by-side or offset bicortical screw placement, the plate having acombination of screw holes and slots.

The “Acromed” system includes a titanium plate and screws which requirebicortical screw placement. This system does not include any lockingmeans for the bone screws.

The system disclosed in U.S. Pat. No. 5,180,381 includes an “H” shapedplate having a combination of ramped slots and a hole which requiresbicortical screw placement at a 45N angle to the plane of the plate.This patent discloses that this angular positioning is for the purposeof producing compression.

The SYNTHES Morscher plate system employs hollow, slotted screw heads.The screws are placed unicortically so that the heads, when properlyaligned, come to rest in the upper portion of the plate holes. The upperportion of each screw is internally threaded to receive a tiny screwwhich is screwed into the bone screw head in order to increase theinterference fit between the bone screw head and the wall of theassociated plate hole.

In the system disclosed in U.S. Pat. Nos. 5,364,399 and 5,423,826, useis made of pairs of unicortical bone screws that may be locked in placeat both ends of the associated plate by locking screws which have asmall diameter shank and a large head. At each end of a plate two bonescrews may be locked in place by a single locking screw which issituated between the bone screws. Generally, the plate is provided,between its two ends, with a diagonal slot or slots for receiving one ormore additional screws, each additional screw being securable in a bonegraft or a respective vertebra which is spanned by the plate. There isno locking screw associated with these intermediate bone screws to lockthe bone screws to the plate.

The Codman Shurtleff plating system utilizes the side of a preinstalledrivet having a head rotatable to press against the side of the head of abone screw so as to secure that one screw to the plate. The plates ofthis system also are provided with holes for receiving intermediatescrews, but these screws are not associated with any locking means.

While the designers of the last-mentioned systems recognized theimportance of locking the bone screws in position on their associatedplates, they did not provide for any locking of the intermediate bonescrews in their associated holes.

In an earlier version of the Codman Shurtleff system, the lockingmechanism was a lever pivotable about a shaft passing entirely throughthe plate and then flared so as to retain the shaft within the plate.The lever was rotated after the bone screw had been inserted to engagethe head of the bone screw and thus secure the bone screw to the plate.

Based on a consideration of the features of all of the known cervicalplating systems, it appears that there remains a need for an improvedsystem having the following combination of features:

-   -   1) The plate should be sufficiently strong to perform its        intended function without mechanical failure;    -   2) The plate should be preformed in three dimensions so as to        anatomically conform in both the longitudinal and transverse        planes to the anterior cervical spine;    -   3) The plate should be constructed so that all of the bone        screws are generally perpendicular to the plate when viewed from        the side, but pairs of screws are highly convergent        corresponding to any vertebral level when viewed from the        bottom, or on end;    -   4) Each pair of screws engages in a respective vertebra and the        high convergence of screws in a pair allows the length of the        screws which engage the bone to be longer and still remain        within that vertebra and provide a safer and stronger engagement        with the vertebrae;    -   5) The system should include bone screws which are capable of        achieving enhanced purchase within the bone of the vertebral        body and without the need to penetrate the posterior vertebral        cortex and enter the spinal canal;    -   6) Use should be made of a screw which is self-tapping, thereby        eliminating the need for separate tapping steps;    -   7) A reliable means should be provided for engaging and        manipulating the plate during installation;    -   8) The plate should be engageable with an instrument means which        can reliably produce bone screw holes which are coaxial with the        screw holes in the plate;    -   9) It should be possible to prepare the vertebral bone to        receive the bone screws so as to produce a stronger connection        and a reduced danger of thread stripping by means of a pilot        hole punch creating a pilot hole for the bone screws;    -   10) Alternatively to the use of a pilot hole punch, a relatively        (compared to the overall root diameter of the screw) small        diameter drill may be used to create the pilot hole.    -   11) Means should be provided for locking each and every bone        screw in position relative to the plate, and the locking means        should be of sufficient size and strength to reliably perform        its intended functions;    -   12) Bone screw locking means should preferably be retainable by        the plate prior to bone screw insertion, or should be reliably        attachable to a driver to prevent any small parts from becoming        loose in the wound; and    -   13) The system should be capable of effecting compression of the        vertebral segments to be fused while maintaining and/or        restoring lordosis.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide an improved anteriorcervical plating system, installation instrumentation, and installationmethod which has the above described features and which avoids many ofthe shortcomings of previously known systems.

One object of the present invention is to provide a locking mechanismwhere a plurality of bone screws used for attaching the plate to thevertebrae can be easily and reliably locked in place at the same time bya single operation.

Another object of the present invention is to provide a vertebral platein which the locking mechanisms for locking the bone screws may bepre-installed by the manufacturer prior to the insertion of the bonescrews by the physician so that the physician does not have to attachthe locking mechanism to the plate as a separate procedure during theoperation.

Another object of the invention is to provide an anterior cervicalplating system which allows for the intersegmental compression of thespinal segment (compression of the adjacent vertebrae and the fusiongraft in the disc space between the adjacent vertebrae) in lordosis, andsimilarly, where desired, multisegmental compression.

A further object of the invention is to provide bone screws whichprovide for tactile feedback to the surgeon to assure sufficienttightening of the screws while avoiding stripping and are less prone tofailure by breakage or by loosening.

Another object of the invention is to provide bone screws which achieveoptimal purchase within the bone, without the need to penetrate theposterior cortex of the vertebrae.

A further object of the invention is to provide plates which aretextured or otherwise treated to promote bone growth from vertebrae tovertebra beneath the plate.

Another object of the invention is to provide a plate which isconstructed to reliably engage an instrument for forming all bone screwholes coaxial with the holes formed in the plate, the instrument havingintegral depth limiting means which completely eliminates the danger ofperforation of the posterior vertebral wall or entry into the spinalcanal.

Yet another object of the invention is to provide a system in which thebone screws and locking mechanisms, when fully installed, have a lowprofile.

It is another object of the present invention to provide for an anteriorcervical plating system which is at least in part bioresorbable.

It is another object of the present invention to provide for an anteriorcervical plating system comprising at least in part of bone ingrowthmaterials and surfaces.

It is another object of the present invention to provide for an anteriorcervical plating system comprising at least in part of bone growthpromoting substances.

It is another object of the present invention to provide instruments forreliably and easily performing the installation of the plates of thepresent invention.

It is still another object of the present invention to provide animproved method of installing the plates of the present invention.

The above and other objects and features of the invention will becomemore readily apparent from the following description of preferredembodiments of the invention, provided with reference to theaccompanying drawings, which illustrate embodiments of the inventionsolely by way of non-limiting example.

SUMMARY OF THE INVENTION

The plating system of the first preferred embodiment of the presentinvention comprises a plate having a length sufficient to span a discspace and to overlap, at least in part, at least two adjacent cervicalvertebrae, a substantial portion of the lower surface of the platepreferably being biconcave, that is concave curved along a substantialportion of the longitudinal axis of the plate and concave curved along asubstantial portion of the transverse axis of the plate. The lowersurface of the plate may also textured and/or treated to induce bonegrowth along the lower surface of the plate which contacts the cervicalvertebrae. The plate is provided with a plurality of bone screwreceiving holes which extend through the plate, from the upper surfaceto the lower surface of the plate, and at least one locking element isassociated with the bone screw receiving hole. The plate and itscomponent parts, may be made of any implant quality material suitablefor use in the human body, and the plate and associated component may bemade of a bioresorbable material.

Bone screws are each insertable into a respective bone screw receivinghole for attaching the plate to a vertebra. A locking element, isengageable to a locking element receiving recess and has a head formedto lock the bone screws to the plate. In the preferred embodiment, asingle locking element locks a number of different bone screws in place.The locking elements are pre-installed prior to use by the surgeon in amanner so as to not impede installation of the bone screws.

As a result, the problems previously associated with the locking screwsof the type applied after the insertion of the bone screws, includingthe problems of instrumentation to position and deliver to the plate thelocking means, backing out, breakage, stripping and misthreadingassociated with the prior art more delicate locking screws resembling“watchmaker's parts”, are eliminated.

In an alternative embodiment of the present invention, a locking elementfits within a respective bone screw receiving hole to lock a respectiveone of the bone screws in place. According to this second embodiment ofthe invention, each of the bone screws is locked to the plate by meansof an individual locking element which bears against at least a portionof the bone screw. Since no other holes need be formed in the plate toattach the locks to the plate, the plate remains quite strong.

The locking elements can be in many forms to achieve their intendedpurpose, such as, but not limited to, screws, threaded caps, rivets, setscrews, projecting elements, and the like.

Also, a novel bone screw is disclosed so as to prevent pulling out ofthe bone screw during use. This is achieved by a design which includes ascrew in which the outer diameter or crest diameter of the thread ismaintained substantially constant along the entire length of the shaftof the bone screw, from below the head to above the tip, where threadsof a lesser outer diameter facilitate insertion. The screw tip is flutedat its distal end to be self-tapping. The thread also has an extremelythin and sharp profile to cut into and preserve the integrity of thevertebral bone stock.

The plating system does not require that the head of the bone screw behollow, or that additional holes be placed through the plate in additionto those provided for the passage of the bone screws. It will beappreciated that bone screws are weakened when their heads are hollowand that plates are weakened when they are provided with additionalholes.

Additionally, the plate of the disclosed systems permit the properaligning of the holes in the plate for the bone screws and for the plateto be easily applied to the vertebrae in compression. The plates includeappropriate slots and engagement means for engaging compressioninstrumentation, described in detail below, for applying a compressionforce between adjacent vertebrae to which the plate is attached, in areliable and easy manner.

An improved locking screw driver is provided. The driver provides for awedged interference fit with a recess in the head of the bone screws andthe head of the locking elements. The same driver is usable for bothbone screws and locking elements. The driver ensures that the lockingelement cannot fall off the driver and become lost in the wound. Thedriver has a tapered end to facilitate insertion into the complimentaryrecess in the head of the screws and is used to engage and pick up thelocking elements. Alternatively, the receiving socket can be tapered tothe same purpose.

Alternatively, a combination bone screw and locking screw driver isdisclosed in which the bone screw driver passes through a longitudinalopening in the locking screw driver so that both the bone screw and thelocking screw can be loaded prior to insertion of the bone screw andboth can be tightened with one instrument, without removing it fromposition.

Also, instruments are provided for forming pilot holes to assist in theease and accuracy of the installment of the bone screws, and forcreating a creating a compression force between adjacent vertebraeduring installation of the plate and for holding the plate duringinstallation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a first embodiment of a cervicalspine multiple locking plate.

FIG. 2 is a top plan view of the cervical spine multiple locking plateshown in FIG. 1.

FIG. 3 is a side elevational view of the cervical spine multiple lockingplate shown in FIG. 1.

FIG. 4 is an end view of the cervical spine multiple locking plate shownin FIG. 1.

FIG. 5 is a bottom plan view of the cervical spine multiple lockingplate shown in FIG. 1.

FIG. 6 is a top plan view of the cervical spine multiple locking plateshown in FIGS. 1–5, with locking elements installed in an openconfiguration.

FIG. 7 is a top plan view of a modification of the plate of FIGS. 1–6with a four bone screw locking element in place.

FIG. 8 is a top plan view of a further embodiment of a cervical lockingplate of FIG. 1 with an elongated central slot for increased compressioncapability.

FIG. 9 is a top plan view of a locking element for use with the platesof FIGS. 1–6.

FIG. 10 is a top plan view of a locking element for use with the centralopening of the plate of FIGS. 7 and 22.

FIG. 11 is a top plan view of a locking cap for use in the end openingsshown in FIGS. 1, 6, and 7.

FIG. 12 is a side elevational view of the locking element of FIG. 16.

FIG. 13 is a side elevational view of another embodiment of the lockingelement of FIG. 16.

FIG. 14 is a top perspective view of an alternative embodiment ofcervical spine multiple locking plate for use with locking rivets.

FIG. 15 is a bottom plan view of the cervical spine multiple lockingplate of FIG. 14.

FIG. 16 is a top plan view of a two bone screw locking element.

FIG. 17 is a top plan view of an alternative embodiment of a four bonescrew locking element having head slits for increased flexibility of thelocking tabs.

FIG. 18 is a bottom plan view of a rivet type locking element for usewith the central opening of the plate of FIG. 14.

FIG. 19 is a side elevational view of a rivet locking element.

FIG. 20 is a top perspective view of the bottom portion of the head ofrivet of FIG. 19 viewed along lines 20—20.

FIG. 21 is a top perspective view of the head portion of a three bonescrew locking element.

FIG. 22 is a top perspective view of a third embodiment of a cervicalspine multiple locking plate utilizing locking elements in the form ofthreaded caps.

FIG. 23 is a side elevational view of a locking element for use with theplate of FIG. 22.

FIG. 24A is a side elevational view of a bone screw in accordance withthe present invention.

FIG. 24B is an enlarged side elevational view of the bone screw of FIG.24A.

FIG. 25 is a side elevational view of an alternative embodiment of abone screw in accordance with the present invention.

FIG. 26 is a bottom end view of the bone screw shown in FIG. 24A.

FIG. 27 is a top end view of the bone screw shown in FIG. 24A.

FIG. 28 is a top perspective view of a fourth embodiment of a cervicalspine multiple locking plate.

FIG. 29 is a top perspective view of a locking element for use with theplate of FIG. 28.

FIG. 30 is a partial side sectional view of the plate of FIG. 28 alonglines 30—30 with a bone screw in place.

FIG. 31 is a top perspective view of the plate of FIG. 1 positionedagainst the anterior aspect of three successive vertebral bodies in thecervical spine, a plate holder, and an instrument for forming bone screwreceiving holes in to the vertebral bodies.

FIG. 32 is a cross-sectional view of a portion of the bone formingdevice shown in FIG. 31 viewed along lines 32—32.

FIG. 33 is a side elevational view in partial cross section illustratinga compression post tool and a compression post engaged to it forinsertion into a vertebral body.

FIG. 34 is a side elevational view in partial cross section of thecompression post tool engaged for removal of the compression post fromthe vertebral body.

FIG. 35 is a bottom end view of the compression post tool of FIG. 34.

FIG. 36 is a side elevational view of a plate engaging hook for use withthe compression apparatus shown in FIG. 38.

FIG. 37 is a cross-sectional view through the plate of an alternativeembodiment of a hole forming instrument in the form of a drill guide anddrill for use during the plate installation procedure.

FIG. 38 is a side elevational view showing intersegmental compression ofthe spine and compression apparatus.

FIG. 39 is a view similar to that of FIG. 38 showing the compressionapparatus in a further stage of the plate installation procedure.

FIG. 40 is a top perspective view showing the locking of the bone screwsto the plate.

FIG. 41 is a partial side sectional view of a locking element attachedto a driver instrument.

FIG. 42 is a partial side sectional view of another embodiment of thelocking element attached to a driver instrument.

FIG. 43 is a partial cross-sectional view showing a cervical plate,locking element, and bone screws along lines 43—43 of FIG. 40.

FIG. 44 is an enlarged portion of detail along line 44 of FIG. 43.

FIG. 45 is a side view in partial cross section of a plate holderattached to a plate.

FIG. 46 is a side view in partial cross section of another embodiment ofa plate holder attached to a plate.

FIG. 47 is a top perspective view of a first embodiment of a singlelocking plate.

FIG. 48 is a top plan view of the plate shown in FIG. 47.

FIG. 49 is a side elevational view of the plate shown in FIG. 47.

FIG. 50 is an end view of the plate shown in FIG. 47.

FIG. 51 is a bottom plan view of the plate shown in FIG. 47.

FIG. 52 is a top plan view of the plate shown in FIG. 47, with lockingelements in place.

FIG. 53 is a side elevational view of a bone screw used with the plateshown in FIG. 47.

FIG. 54 is a top end view of the bone screw shown in FIG. 53.

FIG. 55 is a bottom end view of the bone screw of FIG. 53.

FIG. 56 is a top plan view of a locking cap for use with the singlelocking plate of FIG. 47.

FIG. 57 is a side elevational view of the locking cap shown in FIG. 56.

FIG. 58 is a bottom plan view of the locking cap shown in FIGS. 56 and57.

FIG. 59 is a bottom perspective view of the locking cap of FIGS. 56–58.

FIG. 60 is a top perspective view of the single locking plate of FIG. 47shown being held by a plate holder against three vertebral bodies, withthe hole forming instrument for punching a pilot hole into the vertebralbodies for receiving a bone screw.

FIG. 61 is a side elevational view in partial cutaway of the holeforming instrument threaded to a bone screw receiving hole.

FIG. 62 is a perspective side sectional view of the drill and drillguide threadably engaged to the plate for drilling a hole for insertionof a bone screw.

FIG. 63 is a top perspective view of a single locking plate installedalong a segment of the spine with two locking caps installed in two bonescrew receiving holes.

FIG. 64 is a side elevational view in partial cross section of a lockingcap engaged to a driver for installing the locking cap.

FIG. 65 is a partial cross sectional view of the plate, bone screws andlocking caps along line 65—65 of FIG. 63.

FIG. 66 is an enlarged fragmentary view of area 66 of FIG. 65.

FIG. 67 is a perspective view of a cervical locking plate being held byan alternative plate holder instrument.

FIG. 68 is an end sectional view showing the plate holder of FIG. 67engaging a plate.

FIG. 69A is an end sectional view of an alternative embodiment of theplate holder.

FIG. 69B is an end sectional view of another alternative embodiment ofthe plate holder.

FIG. 70 is a plate holder instrument with an offset and removablehandle.

FIG. 71 is a top perspective view of a second embodiment of a cervicalsingle locking plate having individual locking elements to lock eachbone screw.

FIG. 72 is a top perspective view of a threaded locking element for usewith the cervical single locking plate of FIG. 71

FIG. 73 is a partial side sectional view of the plate of FIG. 71 viewedalong lines 73—73 with the locking element of FIG. 72 in place to hold abone screw, but not fully tightened.

FIG. 74 is a top perspective view of an alternative locking element foruse with a first modification of the cervical single locking plate ofFIG. 71.

FIG. 75 is a side sectional view of the first modification of the plateof FIG. 71 with the locking element of FIG. 74.

FIG. 76 is a perspective view of an alternative locking element for usewith the first modification of the plate of FIG. 71.

FIG. 77 is a partial side sectional view of the first modification ofthe plate of FIG. 71 with the locking element of FIG. 76 in place.

FIG. 78 is a top perspective view of another alternative locking elementin the form of a rivet for use with a second modification of the lockingplate of FIG. 71.

FIG. 79 is a partial side sectional detail view of the plate of FIG. 71modified to use a locking element of FIG. 78 shown in place.

FIG. 80 is a partial cross sectional view of a plate and bone screw withthe end of a tool shown for use in inserting both the bone screws andlocking caps.

FIG. 81 is a side elevational view of another embodiment of the tool ofFIG. 80.

FIG. 82 is a further embodiment of a cervical spine single locking platefor use in stabilizing multiple segments of the spine.

FIG. 83 is a further embodiment of a cervical spine multiple lockingplate for use in stabilizing multiple segments of the spine.

FIGS. 84A–84E are various embodiments of cervical spine multiple lockingplates for use in stabilizing a single segment of the spine.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will be described first in association with thepreferred embodiment of the plate system in which a plurality of bonescrews are locked in place with one locking element. This is referred toas the multiple locking plate system. The multiple locking plates willbe described, then the locking elements for locking the bone screws tothe plate, then the bone screws associated with the multiple lockingplates, and finally the instrumentation and method of installation ofthe multiple locking plates. Thereafter the plate systems in which asingle locking element locks a single bone screw will be described. Thisis referred to as the single locking plate system. The locking elements,bone screws, instrumentation, and method of installation associated withthe single locking plate will then be discussed.

1. Multiple Locking Plate System

The preferred embodiment of the multiple locking anterior cervicallocking plate 2 according to the present invention (here shown by way ofexample for use in a two level fusion (three adjacent vertebrae)) isshown in FIGS. 1–5. Plate 2 has a generally elongated form whose outlinegenerally departs from rectangular due to the presence of lobes orlateral projections 4 at the corners and at the center of the sides ofplate 2. Each lobe 4 has a rounded outline and contains a respectivecircular bone screw receiving hole 6. Two additional intermediatecircular bone screw receiving holes 8 are located inwardly of the sidesof plate 2 and are centered on the longitudinal center line of plate 2.Lobes 4 give plate 2 additional strength in the region surrounding eachbone screw receiving hole 6. It is recognized that other shapes for theplate 2 may be employed.

The intermediate paired bone screw receiving holes 8 are for use with atwo level (three vertebrae) fusion. The intermediate bone screwreceiving holes 8 may be eliminated for a single level (two vertebrae)fusion, or additional intermediate bone screw receiving holes 8 may beadded if additional levels are to be fused.

Plate 2 is further provided with three locking element holes 12, each ofwhich in the preferred embodiment is internally threaded 3, and each ofwhich is surrounded by a shallow countersunk region 14. As will bedescribed in greater detail below, in the preferred embodiment, bonescrews are inserted in the bone screw receiving holes and a singlepre-installed locking element associated with each of the lockingelement holes 12 locks a number of bone screws 30 in position at onetime.

The number of paired bone screw holes generally correspond to the numberof vertebrae to be fused. A plate for a one level fusion could have buta single locking element hole 12, while plates for fusing more than twolevels (three vertebrae) could have additional middle locking elementholes 12 corresponding to additional paired bone screw holes. In theembodiment illustrated in FIGS. 1–6, each end locking element 20 willlock three bone screws 30 in place, while the locking screw 21 in thecentral locking hole 12 locks two bone screws 30 in place. As shown inFIG. 7, central locking element 25 can also be configured so that fourbone screws 30 are locked at one time.

As shown particularly in FIGS. 3, 4 and 5, plate 2 is shaped so that itsbottom surface 27 (the surface which will be in contact with thevertebral bodies) has a biconcave curvature, being concave both in thelongitudinal plane (corresponding to its length) and in the planetransverse thereto, corresponding to its width. The concave curvature inthe longitudinal plane conforms to the proper shape of the anterioraspect of the spine with the vertebrae aligned in appropriate lordosis.That longitudinal curve is an arc along the circumference of a circle(referred to herein as the “radius of curvature”) 15.0 cm to 30.0 cm inradius and more preferably 20.0–25.0 cm in radius. Viewed on end in FIG.4, the plate 2 has a radius of curvature of a circle 15–25 mm in radius,but preferably 19–21 mm in radius. While the plate 2 may have athickness between 2 to 3 mm, a thickness of between 2.25 and 2.5 mm ispreferred.

Having the bottom surface 27 of plate 2 contoured so that it is able tolie flush against the associated vertebral bodies is in contrast toconventional plates which have larger radii of curvature that contactthe vertebral bodies only along the longitudinal centerline of theplate, thereby permitting side-to-side rocking of the plate relative tothe vertebral bodies. The contour of the plate of the present inventionprovides effective resistance to rocking of the plate 2 relative to thevertebral bodies about the longitudinal center line of the plate,thereby reducing stress on the plate 2 and bone screws 30, andpreventing the soft tissues from becoming engaged beneath the plate.

Other advantages produced by the above curvature are that the plate 2will conform more closely to the facing bone surface; the plate 2 willproject from the spine by a smaller distance; soft tissue will beprevented from sliding underneath the edges of the plate 2, where itcould be subject to damage; and the angle of the bone screws 30,perpendicular to the plate when viewed from the side, when installedwill be at a substantial converging angle, trapping the vertebral bonebetween the bone screws 30, and thus more strongly anchoring the plateto the spine.

As shown in FIG. 5, the bottom surface 27 of plate 2, preferably has aporous, roughened, and/or textured surface layer and may be coated with,impregnated with, or comprise of fusion promoting substances (such asbone morphogenetic proteins) so as to encourage the growth of bone alongthe underside of the plate 2 from vertebrae to vertebrae. The texturedbottom surface 27 also provides a medium for retaining fusion promotingsubstances with which the bottom surface 27 layer can be impregnatedprior to installation. The bottom surface 27 of plate 2 may be given thedesired porous textured form by rough blasting or any other conventionaltechnology, such as etching, plasma spraying, sintering, and casting forexample. If porous, the bottom surface 27 is formed to have a porosityor pore size in the order of 50–500 microns, and preferably 100–300microns. Fusion promoting substances with which the porous, texturedbottom surface 27 can be impregnated include, but are not limited to,bone morphogenetic proteins, hydroxyapatite, or hydroxyapatitetricalcium phosphate. The plate 2 may comprise of at least in part aresorbable material which can further be impregnated with the bonegrowth material so that as the plate 2 is resorbed by the body of thepatient, the bone growth material is released, thus acting as a timerelease mechanism. Having the plate 2 being made from a material that isresorbable and having bone growth promoting material present permits thevertebrae to be fused in a more natural manner as the plate becomesprogressively less load bearing thereby avoiding late stress shieldingof the spine.

As further shown in FIGS. 4 and 5, at least one end of plate 2 has arecess 18 that can cooperate with a compression apparatus, described indetail later in reference to FIGS. 36 and 38.

FIG. 6 is a top plan view of the plate 2 of FIG. 1 with locking elements20, 21 inserted into the locking element receiving holes. In thepreferred embodiment, the locking elements 20, 21 are in the form ofscrews that cooperate with the threaded interior 3 of the locking holes12. Each of these locking elements 20, 21 is shown in its initial openorientation, where the orientation of the cutouts 22 in the head 23 ofeach locking element 20, 21 is oriented so as to permit introduction ofbone screws 30 into adjacent bone screw receiving holes 6,8 withoutinterference by the head 23 of the locking element 20, 21. It isappreciated that other configurations of the head 23 are possible so asto permit introduction of bone screw into adjacent bone screw receivingholes without interference by the head 23.

FIG. 8 is a top view of another embodiment of plate 2 of FIGS. 1–5, andis generally referred to as plate 120. Plate 120 is provided with alongitudinally extending elongated slot 122 along its longitudinal axiswhich is superimposed on the middle locking hole 12. Elongated slot 122allows additional relative movement between plate 120 and a compressionpost 54 associated with a compression tool during the compressionprocedure, as discussed below.

Referring to FIGS. 14 and 15, an alternative embodiment of a multiplelocking plate referred to by the number 70 is shown. In plate 70, ratherthan the threaded locking hole 12, a central opening 200 for receiving aremovable rivet 202, of the type shown in FIGS. 17–20, is provided. FIG.15 is a bottom plan view of the plate 70 shown in FIG. 14. The contourof the plate 70 is the same as that of the plate 2 shown in FIGS. 1–5.The rivet 202 is removable and fits within the unthreaded opening 200,comparable to the locking hole 12 and slot 122 described above. Otherembodiments may employ a rivet that is not removable, but ismanufactured as part of the plate 70 as would be used in the end lockingholes 19 of FIGS. 14 and 15.

Referring to FIG. 22, another alternative embodiment of a multiplelocking plate is shown and is generally referred to by the number 230.The plate 230 uses threaded caps, such as cap 300 shown in FIGS. 9 and23, for a locking element or preferably one with cut outs as describedhaving an appearance in a top view such as the locking element in FIGS.10–11, for example. The central locking hole 232 has an elongated slot234 for providing an increased compression capability, as will bediscussed further herein.

Referring to FIGS. 10–13, a first embodiment of a locking element 20,21, 25 in the form of locking screws according to the present inventionfor use with plate 2 is shown. FIG. 10 is a top plan view whichillustrates the head 23 of the central locking element 25 shown in FIG.7. The shaft 46 of locking element 25 is threaded 47 to mate with thethreading 3 within the associated locking hole 12 of plate 2. As shownin FIG. 21, each segment 49 on each side of cutouts 22 of the lockingelement 21 has a bearing surface 48 formed at the lower surface oflocking element head 23. As shown in FIG. 16, the locking element head23 can be provided with two slots 42 for providing flexibility to thelocking element head 23 to assist in the locking element's ability toride over the top of the bone screw head 32 during the bearing actionwhen the locking element is rotated. Alternatively, it is appreciatedthat the bearing surface can be cammed, ramped or wedged. The cammed,ramped or wedged features can also be used with the other lockingelements described herein.

Referring to FIGS. 6 and 10–13, it will be appreciated that when thelocking elements 20, 21 are rotated in the clockwise direction withrespect to the view of FIG. 6, a respective bearing surface 48 (as bestseen in FIG. 21) will ride upon the curved top surface 39 of arespective bone screw head 32 in order to positively lock the associatedbone screws 30 and the locking elements 20, 21 in place.

Alternatively, as shown in FIGS. 12 and 13 in place of a bearing surface48, a ramp or wedge shaped surface 44 may be used to increase the forceapplied to the bone screw head 32. When locked, the leading end of theramped portion of the locking element would be lower than the prominenceof the bone screw head 32 so that more force is needed to lift thelocking element and untighten it than is needed for the locking elementto remain tight and locked. However, the locking element heads 23 neednot have slots, be cammed, or have a ramped surface to achieve thelocking of the bone screw 30 in place. Pressure, friction, interferencefits, or other engagement means capable of preventing the lockingelement from moving from its locked position may be employed.

The rivet 202, shown in FIGS. 17–20 is intended for use in associationwith plate 70 shown in FIGS. 14–15, is shown in detail in cross sectionin FIGS. 19 and 20. The rivet 202 has a head 204, a shaft 206, and anelongated bottom segment 208 for fitting within the correspondingopening 200 in the plate 70. The lower surface 210 of the head 204 ofthe rivet 202 has an irregular surface which may be cammed, such as onthe bottom of locking element 20, 21, for engaging the top surface 39 ofthe bone screw head 32. For use in the end locking holes 19, the uppersurface of the elongated bottom segment 208 can have an irregularsurface for cooperating with the irregular surface of the bottom of theplate 70 to hold the rivet 202 in the locked position against the bonescrew head 32, as shown in FIG. 15. While the rivet of FIG. 18 is aseparate, removable component from the plate, the rivets, andparticularly those for use with the end locking holes, can be formed aspart of the plate during the manufacturing process of the plate andrivet can be non-removable.

Each of the above embodiments provides tight attachment of the lockingelement relative the bone screw 30 and relevant plate.

In the alternative embodiment of multiple locking plate 23 shown in FIG.22, the locking element can be in the form of threaded locking cap 300shown in FIG. 23. The threaded locking cap 300 has a thread 302 on itsouter circumference corresponding to the thread 303 on the innercircumference of the locking element depressions 304 in the top of theplate 230 shown in FIG. 22. The locking cap 300 is relatively thin,particularly compared to its width. The top 305 of locking cap 300 isprovided with a noncircular through hole 306 for receiving a similarlyconfigured driving tool.

Referring to FIGS. 28, 29, and 30 another embodiment of the multiplelocking plate generally referred to by the number 400 and a lockingelement in the form of a thin locking member 412 are shown. Plate 400has an opening in its top surface for insertion of the thin lockingmember 412, a recess 402 associated with each of the bone screwreceiving holes 408 and a slot 410 in the side wall of the bone screwreceiving holes 408 to permit the thin locking member 412, having aseries of thin projections or blades 414, thinner than the slot 410,that give this locking member 412 an appearance similar to that of apropeller. The thin locking member 412 is able to be rotated within theplate so as to not cover the bone screw holes, thus allowing the thinlocking member 412 to be pre-installed prior to the installation of thebone screws by the surgeon. Limited rotation of the thin locking member412 allows the blades 414 to protrude through the slot 410 and to covera portion of the top of the associated bone screws 30. The blades 414 ofthe thin locking member 412 are flexible and, when rotated, slide overthe top surface 39 of the bone screw head 32 to lock the bone screw 30in place. As with the other embodiments discussed, each of theembodiments of the locking element is capable of locking more than onebone screw 30. It is appreciated that the various multiple lockingplates and locking element combinations are capable of locking as manyas four bone screws at once, but are equally effective for locking alesser number or none at all, that is securing itself to the plate.

It will be noted that one characteristic of each of the above describedlocking element embodiments is to have a driver engagement means, inthese cases for example, a recess 24 as large as the recess 34 in thebone screws 30 so that the same tool can be used to turn both the bonescrews 30 and the locking elements. Also, the locking elements aresufficiently strong and have sufficient mass so as to be able towithstand being locked without breakage.

All of the shown examples of the multiple locking elements that have anumber of cutout portions have an arc with a radius greater than that ofthe bone screw head. In addition, the head 23 of each locking element20, 21 is provided at its center with a noncircular recess 24, such asshown in FIG. 9 which is engageable by an appropriate manipulation tool,such as shown in FIGS. 40–42. In the embodiment of head 23 shown in FIG.9, the associated tool would have a hex head, but as discussed withregard to FIGS. 80 and 81, other shapes of recesses in the head 23 maybe used. The thread of each locking hole 12 and of each locking element20, 21 has a close tolerance so that they will reliably retain theirorientations so as to permit introduction of bone screws 30 into bonescrew receiving holes 6, 8 without interference.

It is appreciated that while various forms of locking elements have beendisclosed, in light of the teaching, other equivalent means can be usedfor the purpose of locking the bone screws 30 in place. In FIG. 83, analternative multiple locking plate 990 is shown having additionalintermediate bone screw receiving holes 980 and associated lockingelements 960 for locking bone screws 30 in place. Plate 990 allows for amore close spacing and more pairs of bone screw holes than the number ofvertebrae to be engaged.

In FIGS. 84A–84E various plates 700 a–g used for a single level fusionare shown. Each of these plates 700 a–g is designed to span one spinalsegment consisting of one disc space and two adjacent vertebrae(containing the bone graft), and have bone screws inserted into the endof the vertebrae through the bone screw receiving holes 6 associatedwith the two adjacent vertebrae and then locked in place. As shown inFIGS. 84A–84E, one locking element 710, or two locking elements can beused to lock four bone screws in place. In FIGS. 84A–84E, each of theplates 700 a–e is shown with the locking elements in their openorientation, before being rotated to lock the bone screws.

Each of the above described plates can have the same generally biconcavecontour as already described for conforming to the anterior aspect ofthe spine.

FIGS. 24A and 24B provide a side view of one embodiment of a bone screw30 according to the present invention. FIG. 27 is a top view of the bonescrew 30. At the center of bone screw head 32 is a profiled recess 34which may have the same form as the recess 24 of each locking element20, 21 in which case it may be turned with the same tool as thatemployed for turning locking elements 20, 21. It is appreciated that thedriver engaging portion of the bone screw 30 could be slotted, and beeither male or female (as is shown).

In the embodiment of bone screw 30 shown in FIGS. 24A and 24B, the bonescrew head 32 is stepped, with the first lower head portion 35 beingcontiguous with the screw shank 33 and has a smaller diameter than theupper portion of the bone screw head 32. When this embodiment of bonescrew 30 is employed, each bone screw receiving hole 6, 8 of the plate 2has a countersunk region 14 matching the diameter of the upper portionof the bone screw head 32 and dimensioned for an interference fit. Thelower portion 35 of the bone screw head 32 is dimensioned to achieve aninterference fit with its associated portion of bone screw receivingholes 6, 8. The larger diameter upper portion of bone screw head 32assures that the bone screw 30 cannot be advanced completely throughbone screw receiving holes 6, 8 of plate 2. The bone screw 30 passescompletely through the upper surface of the plate 2 without engaging theupper surface in any way.

As shown in FIG. 44, the head 32 of screw 30 passes unobstructed throughthe upper surface of the plate until the lower surface of enlarged screwhead 32 engages the upper face of the narrowed bone screw receivingportion at the midsubstance or below the midsubstance of the plate. Thisis considered optimal for allowing for the greatest screw to platestability, even absent the lock, against all forces except those reversethe path of insertion, while still providing for the greatest platestrength beneath the bone screw head 23. That is, since the plate is ofonly generally 2–3 mm in thickness, a sheer vertical circumferentialwall is best able to constrain the motion of a screw if the head issimilarly configured and there is little tolerance between them. Placingthe support of the head near the mid thickness of the plate is preferredas it allows the head to remain large to accommodate the recess for thedriver without being weakened, while placing the support of the headaway from the upper surface of the plate allows the screw head to bedeep into the plate. Placing the support of the head at approximatelythe mid thickness of the plate assures plenty of plate material beneaththe head to support while providing adequate head length above and belowthe contact point to prevent the contact point from acting as a fulcrumby providing adequate lever arms to prevent unwanted motion.

In the alternative embodiment of bone screw 30′, as shown in FIG. 25,bone screw head 32′ is tapered in the direction from the top of the bonescrew head 32′ toward screw tip 36′. Again, the bone screw head 32′ isdimensioned to achieve an interference fit in the associated bone screwreceiving hole 6,8 when the bone screw 30′ has been fully installed.When this embodiment of bone screw 30′ is employed, bone screw receivingholes 6, 8 need not be provided with a countersunk region 4.

In each of the above embodiments of the bone screws, the bone screws 30and 30′ present a unique combination of a tapered screw shaft 33 and ahelical thread 31. The diameter of screw shaft 33 generally increasesfrom a distal portion of the shaft near the screw tip 36 toward theproximal portion of the shaft near screw head 32. In the preferredembodiment, the rate of increase in diameter is also greater near thebone screw head 32. Such a shape avoids stress risers and providesincreased strength at the screw-plate junction, where it is needed themost. The tapering of screw shaft 33 may have a concave form, as shownin FIG. 24A, or may be linear. The distal portion of the screw shaft 33may assume a constant diameter.

Referring again to FIGS. 24A and 24B, the thread 31 of the bone screw 30has a substantially constant outer, or crest, diameter “d” from theproximal portion of the shaft below the bone screw head 32 to the distalportion of the shaft near the bone screw tip 36. In the screw tip 36,the crest diameter of thread 31 may be reduced for preferably one to twoturns to facilitate the insertion and penetration of the bone screw 30into the bone.

In the preferred embodiment, the thread 31 of each bone screw 30 has anouter diameter slightly smaller than the diameter of the lowest portion35 of the bone screw head 32, which is adjacent the trailing, or upper,end of the associated thread 31. In addition, the thread 31 isrelatively thin, in the direction of the longitudinal axis of the screw,and tapers outwardly, and has a cross section of a triangle.

An example of the dimensions of a bone screw for use in human anteriorcervical spinal surgery for insertion into the vertebrae is as follows:the threaded portion of said screw has a length from about 10 mm toabout 22 mm (12–18 mm preferred) and a head length from about 1 mm toabout 3 mm (2–2.5 mm preferred). The threaded portion should have amaximum outside diameter from about 3.6 mm to about 5.2 mm (3.8–4.5 mmpreferred) and the head has a diameter from about 3.8 mm to about 6 mm(4–5.5 mm preferred). The thread pitch is from about 1.25 mm to about2.5 mm (1.5–2.0 mm preferred) and has a sharp and thin threaded profile.The apex of the two faces of the thread have an angle of less than about21 degrees (15 degrees preferred) and the base of the thread is lessthan about 0.60 mm thick (0.25 mm–0.35 mm preferred). The screw has aroot diameter that increases from proximately above the tip of theshank, along the longitudinal axis to proximately below the head portionof the screw. Preferably, the tip of the screw tip is fluted by at leastone cut out section so as to make the screw self-tapping.

Even though the thread 31 of the bone screw 30 has a thin profile, thethread will nevertheless be stronger than the bone into which it isintroduced so that this thread will efficiently cut a thin helicalgroove in the bone tissue. The volume of bone that will be displaced bythe thickness of the thread is minimized by the thin form of the thread,yet the substantial crest diameter of the screw thread maximizes thesurface area of the threads in contact with the bone. While enlargingthe screw shaft 33 diameter near the bone screw head 32 increases itsstrength where needed, reducing the screw shaft 33 diameter away fromthe bone screw head 32 where such strength is not required allows forthe maximum area of engagement for the thread 31 to the bone.

In the preferred embodiment, as shown in FIGS. 24A and 26, bone screwtip 36 is provided with cutting flutes 38, to make the bone screw 30self-tapping. Unlike the prior art bone screws, used for anteriorcervical spinal surgery which are not self-tapping, the thread form ofthe present invention screw is itself more like a tap than aconventional screw in that the threads are very sharp and fluted.Additional embodiments of the bone screws 30 is shown in FIGS. 53–55.

By way of example, plates for fusing three adjacent vertebrae (2interspaces, or two spinal segments) are shown. Each set of the bonescrew receiving holes associated with a vertebrae is considered to be asegment of the plate so that for example, in FIG. 1 three segments areshown—an upper, a central, and a lower segment. While the presentdiscussion is in association with plates for use in fusing threevertebrae across two interspaces, it should be understood that longerand shorter plates having the appropriate number and location of bonescrew receiving holes corresponding to the number of vertebrae to befused are contemplated, and would take the form of the plates shown withfewer or more intermediate segments, such as the segment along line 9 ofFIG. 1, or the intermediate segments of the plates shown in FIGS.82–84F.

Referring to FIGS. 31–42, an outline of the steps of the method forinstalling the plates of the present invention is set forth below. Adetailed description of the instrumentation and method for installingthe plates of the present invention follows the outline.

Step 1

Having completed the interbody fusions, the surgeon removes any bonespurs or localized irregularities along the front of the spine of thearea to be fused.

Step 2

The correct length plate is selected by the surgeon by measuring thedistance on the spine by a caliper, ruler, template, and the like. Thatplate having a length sufficient to span the distance of the spine to befused and to partially overlap a portion of each of the end vertebrae tobe fused.

Step 3

Utilizing a plate holder, the plate is placed into the wound andpositioned to confirm positioning, length, and screw hole alignmentrelative to the segments of the spine to be fused.

Step 4

As shown in FIG. 31, with the plate thus positioned and securely held,the plate may be attached to any of the vertebrae to be fused (byexample only, here shown as the top vertebra).

Sub-Step 4A

The pilot (guide) hole punch 60 is attached to the plate 2 as per FIG.32, or alternatively, while not preferred the drill guide may be used asper FIG. 37. In either event, the pilot hole forming means rigidlyaligns with and is captured by the plate bone screw receiving hole wall.

Sub-Step 4B

The pilot hole is then formed by impacting the pilot hole punch of FIG.32 or drilling with the drill of FIG. 37. In the alternative while notpreferred, the formation of the pilot hole can be done away withaltogether and the correct screw selected so as to have a length lessthan the distance along its path to the posterior vertebral cortex canbe directly inserted.

The determination of the appropriate screw length is made by measuringor templating from radiographs, MRI's, or CT scans, or determineddirectly by measuring the depth of the disc space.

Step 5

The correct screw is then attached to the screw driver which regardlessof the specific form of the screw driver engagement means, is designedto have an interference fit so as to remain firmly bound to the driverduring transport to the insertion site. FIGS. 41, 42, 63, 64, 80 and 81show various ways of achieving such a fit of the driver and screw. Inaddition to a wedging at the screw and driver interface, clips, andsprings and other means are well known for temporarily and reversiblysecuring the screw to the driver, such as is shown in FIG. 80 where aslotted inwardly springing sleeve holds a threaded cap peripherallyuntil, as it is screwed into the plate, it is automatically pushed backreleasing the threaded cap.

Once a first bone screw has been fully inserted into a vertebra throughthe plate, it is preferable to insert the other of the transverse pairin the manner already described as per FIG. 33.

In a similar manner, it is possible to insert the remaining bone screwsas per the surgeon's preference into each of the vertebrae to beincluded into the fusion, just the end vertebrae of the fusionconstruct, or additionally place screws into the fusion grafts.

However, as shown in FIGS. 33, 34, 38 and 39, it is possible with thepresent invention at the surgeon's option to place any portion or all ofthe fusion construct under compression and to do so intersegmentally oracross the entire length of the fusion construct even whenmulti-segmented.

It is appreciated that the same procedure could be generally used forany of the plate systems of the present invention.

As shown in FIG. 31, the vertebrae 50 a–c are separated from one anotherby fusion graft blocks 51 which were previously installed in the spinaldisc space between adjacent vertebrae 50 forming a fusion bone graftconstruct. Plate 2 is shown in FIG. 31 with the locking elements 20, 21removed in order to simplify the illustration. It will be understood,however, that in the preferred embodiment the locking elements 20, 21can be, and preferably are, pre-installed in the positions shown in FIG.6 prior to positioning plate 2 upon vertebral bodies of the vertebrae50, thereby saving the surgeon time and trouble.

Plate 2 may be held in position by any known plate holding means, butpreferably by the holding tools shown in FIGS. 45, 46 or 70 by thenotches 142 in the sides of the compression arms 104, 130 of a vertebralcompressor tool 100 shown in FIG. 39, or as a further alternative, bythe unitary plate holder similar to the FIG. 70 design.

As shown in FIG. 45, plate holder 870 has a hollow tubular housing 872,with a central rod 874 having a thread 878 at one end for engaging oneof the threaded locking holes 12 in the plate 2. The bottom end of thehousing 872 has projections 880, 882 that extend outwardly and thendownwardly to fit into the bone screw receiving holes 8 of the plate 2preventing the housing 872 from rotating. The central rod 874 is locatedin the housing 872 such that it can be rotated by rotating a handle (notshown) which is fixed to the central rod 874 at its upper end.

In FIG. 46 an alternative embodiment of the plate holder 890 is shown. Asingle solid member 890 has a threaded projection 894 at its bottom endfor attachment to the central threaded locking hole 12 in the plate. Thebottom surface of the holder 890 of this embodiment is contoured so asto match the contours of the top surface of the plate adjacent to thelocking hole 12, shown as a depression 14 (FIG. 1).

Referring to FIGS. 67–68, an embodiment of a plate holder for holdingany of the plates while being positioned on the vertebrae is shown andgenerally referred to by the number 800. The plate holder 800 has ahollow tubular housing 802, with a central rod 804 having a handle 806at one end and a thread 808 at its other end for engaging one of thethreaded locking holes 12 in the plate 600. The bottom end of thehousing 802 has projections 810, 812 that extend outwardly and thendownwardly 814, 816 to fit along the side edge of the plate 2 betweenthe end and intermediate lobes 4, preventing the housing 802 fromrotating. The central rod 804 is located in the housing 802 such that itcan be rotated by rotating the handle 806 which is fixed to the centralrod 804 at its upper end. This central rod 804 can also be attached tothe housing 802 so that it can move up and down to some extent, by anynumber of conventional ways, such as by having the central rod 804 havean annular depression with a length of approximately 3–5 mm, and a setscrew projecting inward from the housing to engage the central rod 804.Once the plate 600 is in the proper place and the plate is attached toone of the vertebrae by bone screws 30, the central rod 804 isdisconnected from the opening in the plate 600 and the holder 800 isremoved.

FIG. 69A is an alternative embodiment of the plate holder 850. A singlesolid member 852 has a threaded projection 854 at its bottom end forattachment to the central threaded locking hole 12 in the plate. Thesolid member 852 could also be threaded into a bone screw receiving hole6. The bottom surface of the holder 850 of this embodiment is contouredso as to match the contours of the top surface of the plate adjacent tothe locking hole 12, shown as a depression 14 (FIG. 1).

FIG. 69B is another embodiment of the plate holder 850′. A housing 851′having an end 853′ configured to engage a bone screw receiving hole 6contains a rod 855′ having an uneven diameter and having a threadedportion 857′. As rod 855′ is rotated by a handle similar to handle 806shown in FIG. 68, rod 855′ screws downward into the housing 851′ intomatching threads 858′. As the end of rod 855′ is driven down, it spreadsportions 859 a′ and 859 b′ (859 c′ and 859 d′ not shown) wedging plateholder 850′ into a bone screw receiving hole of the plate. Plate holder850′ is best used with non-threaded bone screw receiving holes, butworks for all types of bone screw receiving holes.

Referring to FIG. 70, an alternative embodiment of the plate holderreferred to by the number 800′ is shown in which there is a removablehandle 860 that is used for first attaching the plate holder 800′ to theplate, by rotating the shaft 804, and then for holding the plate holder800′ off to the side by extension 864, during the attachment procedurereducing the interference of the plate holder 800′ with the surgicalprocedure.

Referring to FIG. 38, a compression tool 100 is shown with a toothedgear bar 102 having a first compression arm 104 secured to its free end.Compression arm 104 has at its distal end a bore 106 for removablyholding either a plate engaging element 108, shown in FIG. 36, having ahook 110 atone end for engaging a depression or notch 18 in the end ofplate 2, or for removably holding a compression post 54 shown in FIGS.33–34. As shown in FIG. 36, plate engaging element 108 includes a shaft112 that will be inserted into the corresponding bore 106 of compressionarm 104, and a flange 115 for resting against the bottom face of bore106 to accurately limit the depth of insertion of plate engaging element108 into the bore 106. A ring spring 128, preferably of metal, islocated in an annular depression of the shaft 112, for holding the plateengaging element 108 in the bore 106.

Referring to FIGS. 38–39, compression tool 100 includes a secondmoveable compression arm 130 movable along toothed bar 102 parallel tofirst compression arm 104. The distal end of the second compression arm130 also has a bore 132, the same as bore 106, that can receive aremovable compression post 54. Bores 106 and 132 are the same so thateither compression arm 104, 130 can be used to hold the removablecompression post 54, permitting the compression tool 100 to be used inany orientation. By permitting the plate engaging element 108 and thecompression post 54 to both rotate and slide in the bores 106, 132 ofthe two compression arms 104, 130, with the plate engaging hook 110 ableto work even at an angle to the plate allows for the apparatus to bereadily attachable to the spine through the compression post 54 andplate.

Compression arm 130 has a driving assembly consisting of a toothed wheel(not visible) which is engaged with the tooth gear 138 of bar toothedgear 102 and is connected to compression arm 130 such that compressionarm 130 is movable along the length of toothed gear bar 102 by means ofthe rotation of handle 140, which is connected to the toothed wheel.When the handle 140 is turned in the direction of the arrow shown inFIG. 38, compression arm 130 is moved toward compression arm 104. Thedriving assembly has a self lock release mechanism whereby the movementof the two compression arms 104, 130 away from one another is prevented,without the activation of the release. On the inward distal end of eachcompression arm, on facing sides, is a notch 142 or recess for holdingthe plate 2 along its sides between the central lobes 4 and end lobes 4,as shown in FIG. 38.

While the toothed gear bar 102 and compression arms 104, 130 have beendescribed as being straight, it is possible that the toothed gear bar102 and compression arms 104, 130 may be arcuately or otherwise shaped,so as to induce lordosis in the vertebrae, if so desired.

As shown in FIG. 31, in the event that the compression tool 100 is usedto hold the plate 2, the ends 144 of the compression arms 104, 130 willbe located in line with the fusion graft construct 51 which was placedin the disc space when plate 2 is properly positioned. A gap will existbetween plate 2 and each fusion graft construct 51, providing a space toaccommodate the free ends of arms 104, 130 should they extend beyond thebottom surface of the plate 2. As will be described below, the samecompression tool 100 can also be used for compressing a plurality ofcervical vertebral bodies with bone grafts interposed during theattachment of plate 2 to the vertebrae 50.

Referring to FIG. 31, plate 2 is held by a suitable holder, in this caseshown as the compression arms 104 and 130. Once the appropriate lengthplate 2 has been properly positioned so that the bone screw receivingholes 6 are aligned with each of the respective vertebrae 50 a–c to befused, the next step is the formation of bone screw receiving holes 6prior to installation of the bone screws 30 themselves in the vertebrae50 a. While the procedure is described as first attaching the plate 2 tothe upper vertebrae 50 a, the plate 2 can be attached to any of thevertebrae in any order. Different sized plates are used so that, asindicated above, the physician will select the appropriate sized platein which the bone screw receiving holes 6, 8 are aligned with the threeadjacent vertebrae 50 a, 50 b and 50 c. Pilot holes are formed by apilot hole forming apparatus 60 shown in FIGS. 31 and 32. Unlike withknown prior art and screw plating systems, the bone screws 30 may beinserted without the prior formation of an opening into the vertebrae asthe bone screws 30 are preferably sharp pointed, self-tapping, and haveat their tip a diminishing major diameter to assist the screw enteringand pulling into the bone. However, while a hole into the bone of thevertebrae may be formed prior to screw insertion, it is preferable thatthe hole be of a smaller diameter than the root diameter of the screwand for a different purpose than with the prior art. With the prior artthe hole drilled had to be of a diameter equal to but preferably largerthan the root (minor) diameter of the screw, as the screws were notself-tapping. It is desirous to create pilot holes to assure that aproper path for the bone screws 30 is maintained, and also to preventdamage to the vertebral bone during insertion of the bone screws 30. Inaddition, the pilot hole forming apparatus 60 creates a more compactvertebral bone mass for reception of the self-tapping bone screw 30 usedin this insertion.

As shown in FIGS. 31 and 32, pilot hole forming apparatus 60 includes ahollow cylindrical housing 62 having a bottom provided with a throughhole 63. Housing 62 contains a central shaft 64 which extends throughthe through hole 63 in the bottom of housing 62. The leading end 66 ofshaft 64 tapers gradually to a sharp point 65. Shaft 64 is provided witha ring member 78 having a diameter which closely corresponds to theinner diameter of housing 62 to guide the travel of shaft 64 withinhousing 62. A compression spring 67 is interposed between the ringmember 78 and the bottom of housing 62. Compression spring 67 provides abias force which normally urges the sharp point 65 into a retractedposition within housing 62. The upper end of shaft 64 has an enlargedhead 68 extending outside of the housing 62 which is intended to bemanually depressed or struck by a percussion instrument in order todrive the sharp point 65 out of housing 62 and into a vertebral body 50a. Shaft 64 is given a length, taking into account the length thatspring 67 will have when fully compressed, to determine the maximumdepth of the pilot hole formed in a vertebral body. The depth isselected to assure that the pilot hole does not reach the posteriorcortex of the vertebral body, which borders the spinal canal.

Certain structural features of hole forming apparatus 60 are shown ingreater detail in FIG. 32. In particular, it can be seen that the bottomend of housing 62 has a projecting portion 69 dimensioned to fitprecisely in a bone screw receiving hole 6 or 8 of plate 2. The bottom71 of the projecting portion 69 is flat in a plane perpendicular to theaxis of housing 62. When the projecting portion 69 of housing 62 issnugly inserted into a bone screw receiving hole 6, 8 and the flatbottom 71 is placed flush against the upper surface of plate 2, it isassured that the leading end 66 of shaft 64 will form a pilot hole inthe vertebral bone having an axis perpendicular to the plane of theassociated portion of plate 2, thereby assuring that the bone screw 30will be subsequently installed so that its axis is also perpendicular tothe plane which is parallel to the upper and lower surfaces of theassociated portion of plate 2.

When a plate is used which has a threaded bone screw receiving hole, thelower end of the pilot hole forming apparatus 60 is threaded so as toengage the thread in the bone screw receiving hole 6, 8 thereby fixingthe plate and the pilot hole forming apparatus together, assuring astable fit between the pilot hole forming apparatus and the plate 2. Itshould be noted that the diameter of the leading end 66 of the shaft 64is small since it has to fit within the small space left between theinside wall of the pilot hole forming apparatus. Since it is only apilot hole for a self-tapping bone screw 30 that is being formed, thesmall diameter is satisfactory.

Referring to FIG. 37, if for any reason it should be desired to form thepilot hole in the vertebral body 50 by drilling, rather than by the useof the pilot hole forming apparatus 60, use can be made of a drill guide80, having a lower end as shown in FIG. 37. The drill 80 guide consistsof a tubular member 82 and a small diameter lower end 84 which isdimensioned to achieve a precise interference fit in the associated bonescrew receiving hole 6, 8 of plate 2. Along the small diameter lower end84, drill guide 80 has an axial end surface in a plane perpendicular tothe longitudinal axis of the drill guide 80 so that when the smalldiameter portion 84 is fitted into the bone screw receiving hole 6 andthe surface surrounding the small diameter portion 84 is flush againstthe upper surface of plate 2, the axis of the drill guiding bore 86 indrill guide 80 will be precisely perpendicular to the upper and lowersurfaces of the associated portion of plate 2. As with the casedescribed above, the bottom end of the drill guide 80 can be threaded soas to engage to the threaded opening of plate 2.

After the bone screw receiving holes 6, 8 are formed in the vertebralbody 50 a through the upper two bone screw securing holes 6 of plate 2by means of either hole forming apparatus 60 or drill guide 80, bonescrews 30 are threaded into the vertebrae 50 while holding the plate 2firmly against the vertebrae 50 with compression tool 100 or plateholder 800. This locks the plate to the vertebrae 50 a.

It is then possible, if desired, to compress the fusion graft in thenext adjacent vertebrae 50 b before attaching bone screws 30 to theadjacent vertebrae 50 b through the central bone screw receiving holesof plate 2. Once the initial bone screws are in place in the vertebrae50 a, the plate holder 100 or 800 may be removed from the plate 2. Thecompression of the fusion graft construct between the two adjacentvertebrae 50 a and 50 b is achieved as follows:

Compression post 54 is driven through the central locking hole 12 ofplate 2 by means of insertion tool 90, shown in FIGS. 33, 34 and 35,into the vertebral bone of vertebra 50 b, where it will be used in asubsequent step to apply a compression force between vertebrae 50 a and50 b. Compression post 54 consists of a shaft 56 having a sharp point 57at its lower end, an enlarged central collar 58 which serves as a depthstop, and a circumferential groove 59 proximate its upper end, definingan enlarged head 55.

Compression post insertion tool 90 consists of a shaft 92 having aclosed hollow portion 94 at its lower end 96 for receiving compressionpost 54 and an enlarged percussion cap 98 at its other end. Compressionpost insertion tool 90 also includes in its lower end 96 a secondopening 95 having a recess 99 in its inside wall for permittingengagement of the enlarged head 55 on the compression post 54 within thedepression 97. The second opening 95 is in communication with the hollowportion 94 of the insertion tool 90, as shown in FIG. 35.

Referring to FIG. 38, the bore 132 in the second compression arm 130 ofcompression tool 100 is then applied over compression post 54 invertebrae 50 b, and the plate engaging element 108 is inserted in thebore 106 of the first compression arm 104 of compression tool 100. Thehook 110 of the plate engaging element 108 shown in FIG. 36 is fittedinto the notch 18 at the end of the plate 2 which is fixed by the bonescrews 30 inserted into the vertebra 50 a, as shown in FIG. 38. Asindicated above, however, the compression tool 100 can be rotated sothat the first compression arm 104 is now at the bottom and is able tofit over the compression post 54 in vertebrae 50 c.

Since the plate is attached to vertebrae 50 a by means of bone screws 30and compression post 54 is fixed to the adjacent vertebrae 50 b,movement of the first and second compression arms 104 and 130 in thedirection of vertebrae 50 a by rotation of handle 140 results incompression of the bone graft construct 51 between the adjacentvertebrae 50 a and 50 b. The distance of several millimeters issufficient for compression of the bone graft construct 51. Once thedesired compression is obtained, bone screw pilot holes can be formed invertebral body 50 b by means of pilot hole forming apparatus 60, asdescribed above, for insertion of bone screws 30 into bone screwreceiving holes 8 of bone plate 2, fixing the plate 2 to the adjacentvertebrae 50 b. Compression tool 100 can then be withdrawn by activationof the release.

FIG. 39 illustrates the use of compression tool 100 to inducecompression between the lower two vertebral bodies 50 b and 50 c afterbone screws 30 have been installed in the middle vertebral body 50 b asjust described. As shown in FIG. 39, compression post 54 remains inplace in the middle vertebral body 50 b and an additional compressionpost 54 is driven into the lower vertebral body 50 c by means of pilothole forming tool 60 distal to the plate itself in the recess betweenthe end projections 4 to allow for the lower compression post 64 to bemoved towards vertebrae 50 b upwardly as shown. The original compressionpost 64 is inserted in bore 106 in the first compression arm 104 and theadditional compression post 54 is inserted into the bore 132 of thesecond compression arm 130 of compression tool 100. Again, as discussedabove, the turning of the handle 140 results in the two compression arms104, 130 moving towards one another, resulting in the compression post54 in vertebrae 50 c moving towards the upper compression post 54 invertebrae 50 b, once again compressing the fusion graft construct 51between vertebrae 50 b and 50 c. The upper compression post 54 invertebrae 50 b can not move since the vertebrae 50 b has been fixed tothe plate by the insertion of the bone screws 30 in the bone screwreceiving holes 8 of the plate 2. Thus, only the lower compression post54 and vertebrae 50 c can move. As before, the pilot holes associatedwith vertebrae 50 c are formed and the bone screws 30 are insertedthrough bone screw receiving holes 6. The compression tool 100 is thenremoved. Compression post 54 is then extracted from the vertebrae byinserting it in the second opening 95 of the compression postinsertion/removal tool 90, so that it engages the enlarged head 55 ofthe end of compression post 54 by depression 97, as shown in FIG. 34.

It is recognized that other variations in the order of compression maybe employed. For example, during the compression of the fusion graftconstruct 51 between vertebrae 50 b and 50 c, the hook 110 of plateengagement element 108 may engage the notch 18 in the end of the plate2, and the other compression arm of the compression tool 100 may engagethe compression post 54 in the third adjacent vertebrae 50 c. It shouldalso be noted that plate 2 has a recess end cut out portion between thelobes at the end of the plate for insertion of the compression post 54in the vertebrae. Otherwise, there may not be room below the end of theplate 2 for insertion of the compression post 54.

It will be noted that the above-described procedure will be performedwith the bone screws 30 fully inserted into vertebral bodies 50 a, 50 band 50 c and lordosis is maintained during compression of the bone graftconstruct 51.

As indicated above, the procedure for attaching the plate 2 to thevertebrae 50 a, 50 b and 50 c was illustrated without the locking screws20, 21 in place on the plate 2. FIG. 40 is a perspective view showingthe plate 2 of FIGS. 1–5, at a stage of a surgical procedure when bonescrews 30 have been fully installed in three adjacent vertebrae 50 a, 50b and 50 c, and locking screws 20, 21 have been rotated through an angleof about 90N to lock three bone screws 30 in place; the left-handlocking screw 20 as viewed has been rotated through an angle of about60N to lock three bone screws 30 in place and the central locking screw21 has been rotated through an angle of about 90N to lock two other bonescrews 30 in place. At this time, one of the camming surfaces 44 of eachlocking screw 20, 21 rests atop the screw head 32 of a respective bonescrew 30.

Installation of the locking cap 300 can also be performed with a tool220 such as shown in FIGS. 41 and 42 having a suitably shaped tip 222with a length corresponding to the depth of hole 306 in a locking cap300. The end 222 of tool 220 is flared just proximal to the most distalend so that it creates a friction fit with the screw cap 300 for ease ofmanipulation, and prevents the screw cap 300 from falling off the tool200.

FIG. 43 is a cross-sectional view in the plane of the center of the twoend locking screw holes 6 of plate 2, with two bone screws 30 in theirinstalled positions and locking element 21 in its locking position. FIG.44 is an enlarged view of one of the bone screws 30 in plate 2 of FIG.43. In a preferred embodiment, the axis of each screw 30 is generallyperpendicular to tangents to the upper and lower surfaces of plate 2 atpoints which are intersected by the longitudinal axis of the associatedbone screw 30. Thus, because of the curvature of plate 2 in the plane ofFIG. 43, bone screws 30 can be directed so as to converge toward oneanother at a desired angle. Preferably, such angle will be greater than14°. More preferably, such angle will be greater than 14° and less than30°. The axis of the two bone screws 30 shown in FIG. 43 may subtend anangle of about 45N. Alternatively, the curvature of the plate from sideto side may be so as to conform to the surface of the anterior aspect ofthe human adult cervical spine and the axis of the paired screw hole maydeviate from being perpendicular to the plate when viewed on end toachieve optimal convergence.

Because the bone screws 30, once inserted, are locked to the plate, a“claw” of a rigid triangular frame structure is obtained at each pair ofbone screws 30 such that the attachment of plate 2 to the vertebralbodies 50 a, 50 b and 50 c would be highly secure due to the trapping ofa wedged mass of bone material between the angled bone screws triangle,even if any thread stripping should occur. The “claw” may be furtherformed by three angled bone screws in a tripod configuration or by fourbone screws in a four sided claw configuration.

A plating system according to each of the above embodiments can beinstalled in the same manner as described above, and using the sameinstruments and tools, as illustrated and described above with respectto the first embodiment. In the case of the embodiment shown in FIG. 22,the compression operations would be performed by means of slot 232instead of the middle locking screw hole 12.

2. The Single Locking Plate Systems

The single locking plate system will now be described. FIGS. 47–52 areviews of a first embodiment of a single locking plate system. Thecontour of plate 600 is the same as the plate 2 shown in FIGS. 1–5.Plate 600 contains bone screw receiving holes 602 which are internallythreaded 603 for receiving corresponding locking elements in the form ofa locking cap 610, shown in FIGS. 56–59. For example, in plate 600, thebone screw hole 602 has an outer diameter of approximately 5 mm with apreferred range of 4–6 mm; and a threaded inner diameter ofapproximately 4.8 mm, with a range of 3.5–5.8 mm for this use. Attachingmeans other than threads may be used, such as bayonet type attachmentelements.

The bottom of each bone screw receiving hole 602 has an inwardly steppedportion of properly selected dimensions for retaining an associated bonescrew 170, as shown in FIGS. 53–55. As described in greater detailbelow, in this embodiment, a single locking element in the form of alocking cap 610 having threads 608 shown in FIGS. 56-59, is associatedwith each of the bone screws receiving holes 602.

The difference between the bone screw 170 used in the single lockingembodiment of the plate from the bone screw used in association with themultiple locking plate is essentially due to the fact that whereas inthe multiple locking plate embodiment the locking elements slide over aportion of the top 39 of the screw head 32, in the single lockingembodiment the locking cap 610 fits over the head 172 of the bone screw170. Therefore, the head 172 of the bone screw 170 of the presentembodiment need not be smooth. This permits the head 172 of thisembodiment bone screw 170 to be thicker and stronger.

FIG. 65 shows two bone screws 170 and associated threaded locking caps610 in their fully installed positions. In these positions, headportions 174 and 176 of each bone screw 170 form an interference fitwith corresponding portions of an associated bone screw receiving hole602. Rim 612 of each threaded locking cap 610 forms an interference fitwith upper portion 178 of the head of its associated bone screw 170.Because the thread 608 of each locking cap 610 mates precisely with theinternal thread in an associated bone screw receiving hole 602, eachthreaded locking cap 610 is additionally subjected to a clamping forcebetween associated head portion 178 and the internal threads 603 ofassociated bone screw receiving hole 602. The rounded head 614 of eachthreaded locking cap 610 assures that the upper surface of an assembledplating system will be free of sharp edges, or projections.

Referring to FIGS. 80 and 81 tools for use in inserting both the bonescrews and the locking cap in the single locking plate 600 are shown. Inthe first embodiment of the driving tool 1000 shown in FIG. 80, the tool1000 has an outer tubular housing 1002. Within the housing 1002 is atorks type or hexagonal driver 1004 that has a projecting end 1006 thatcorresponds to the recess 306 in the cap 610 for engagement with the cap610. As indicated above, the driver 1004 is configured so that it makesa firm attachment for the locking cap 610 for holding the locking cap610 firmly to the driver. The hex driver 1004 is hollow so as to be ableto permit the shaft 1010 of a Phillips or torks screw driver to fitthrough the hollow portion 1012 for engagement by its tip 1012 with thecorresponding recess 180 of bone screw 170 for engagement by the end1006 of the driver 1004. The shaft 1010 of the driver 1000 is longerthan the tubular housing and driver 1004 has an upper end (not shown)extending from the top end of the tubular housing 1002 so that it can berotated by the handle.

The housing 1002 has a diameter that permits the locking cap 610 to beheld within the inner end of the tubular housing 1002 by a friction fitor to the driver 1004. It is appreciated that other methods of holdingthe locking cap 610 within the end of the tubular housing 1000 may alsobe employed.

As shown in FIG. 80, the operation of the bone screw and locking elementdriver 1000 is as follows: the cap 610 is inserted onto the end of thecap driver 1004, and then the cap driver 1004 with the shaft 1010 of thebone screw driver passing through the central longitudinal opening ofthe cap driver. As shown, the bone screw driver shaft 1010 passesthrough the recess 306 in the cap 610 and engages the recess 180 in thehead of the bone screw 170. The bone screw 170 is shown being installedin a bone screw receiving hole in the plate 600. The handle (not shown)of the bone screw driver is rotated, thereby screwing the bone screw 170in place. Since the diameter of the bone screw driver is less than thewidth of the recess 306 of the cap 610, the bone screw driver shaft 1010is able to rotate without rotation of the cap 610.

The hollow tubular housing 1002 rests on the top surface of the plate600 and assists in the alignment of the shaft 1010 in relationship tothe plate. Once the bone screw 170 is inserted, the cap driver 1004 isdepressed until the threads 608 on the outside of the cap 610 engagesthe threads 603 of the bone screw receiving hole. The cap driver 1004 isthen turned until the cap 610 is securely locked in place.

In FIG. 81, an alternative embodiment of the combination bone screw andlocking cap driver is shown. In this embodiment, a housing is not used.Instead, the driver shaft 1010 holds the cap 610 by friction and thehandle 620 for the bone screw driver shaft 1010 is rotated. A ballspring assembly 622 holds the cap driver 1002 up until the bone screwhas been screwed into the bone screw receiving hole. Driver 1010 has anelongated portion that once the bone screw has been installed, the ballspring 622 is depressed and the handle 624 associated with the capdriver is permitted to descend for rotation of the cap 610. A tubularhousing can be employed to assist in aligning of the cap 610 in the bonescrew receiving hole, as indicated above.

The drivers shown in FIGS. 80 and 81 simplify the procedure, and reducethe number of instruments that are necessary to be used during theinstallation procedure. The procedure is quick and reliable, giving thephysician more assurance that small watch parts will not be lost ordifficult to manipulate.

FIG. 52 is a top view of the plate 600 partially installed, withthreaded locking caps 600 installed in bone screw receiving holes 602.

FIGS. 53–55 show a bone screw 170 for use with the single lockingplating system according to the invention. Bone screw 170 differs frombone screw 30 previously described in detail, only with regard to thestepped configuration of head 172. Preferably, bone screw 170 includes alower portion 174 which is contiguous with the screw shank and has areduced diameter equal to the maximum diameter of the shank 176. Portion178 of head 172 also has smaller diameter than lower portion 174. Thethread 182 has the same configuration as for the bone screw 30 discussedabove. However, either embodiment of bone screws can be used with any ofthe plates.

As in the case of the multiple locking plating system described above,the bone screws 170 for use in the single locking plating system arepreferably solid, where the screws adjoin the lower plate surface, wherescrews used with prior art plates are most prone to breakage, the onlyrecess in the heads being for engagement of the tip 222 of driving tool220 and with the recess being above the critical area. Therefore, thesebone screws 170 remain robust. The screw heads are not deeply slittedinto portions and the locking caps do not impose a radial outer force onthe associated bone screw heads so the screw heads do not spread apartso as to be stressed and weakened.

Referring to FIGS. 71, 73 and 75 another alternative embodiment of thesingle locking plate system of the present invention is shown andreferred to by the number 500. The plate 500 has the same contour as theplate 2 shown in FIGS. 1–5, but associated with each of the bone screwopenings 502, are threaded openings 524 offset from the bone screwopenings 502 for receiving the locking element 506, 508, shown in FIGS.72 and 74 as a threaded locking set screw or cap 506 or screw 508.

It is appreciated that other configurations of single locking plates maybe employed. Referring to FIG. 82, a single locking plate 900 is shownin which there are a pair of bone screw receiving holes 910 at its ends930 and a number of bone screw receiving holes 950 along thelongitudinal axis of the plate 900. The additional bone screw receivingholes 950 permit a single plate to be able to be aligned with a numberof different sized vertebrae disc spaces, and bone fusion grafts. Asindicated above, the plate of the present invention shown in FIGS. 1–5,requires that a properly sized plate be selected by the surgeon so thateach pair of bone screw receiving holes 6, 8 line up with theappropriate vertebrae. This requires a number of different sized platesto be available for optimum attachment of the bone screw receiving holesto each of the vertebrae. With the plate 900 of FIG. 82, the closespacing and increased number of central openings permit the surgeon tolocate at least one appropriate opening to be aligned with each of theintermediate vertebrae, and/or bone grafts.

The procedure for installation of the single locking plates issubstantially the same as described herein in detail for the multiplelocking plates. The central longitudinal slot 670 in the single lockingplates is used for the compression procedure. The same instrumentationis used to create the plate hole either by means of a punch or a drill.FIGS. 60–69 show the various steps in the procedure for installation ofthe single locking plates, comparable to the steps employed in theinstallation of the multiple locking plates.

Referring to FIGS. 76–79 the heads 507 and 526 of the locking elements508 and 522 have a recess 510 and 524 corresponding to the radius of thebone screw openings 502 and 528 so that the locking element 508 and 522may be installed in place prior to the insertion of the bone screw 170into the bone screw receiving hole 502 and 528. When the lockingelements 508 and 522 are rotated, a portion of its head extends over thetop of the head of bone screw 170 to lock it in place. As with the aboveembodiments, the bottom surface of the locking screws 508 and 522 canhave a camming or other configuration for engagement with the topsurface 39 of the associated bone screw 170.

While the plate instrumentation and method have been described inassociation with attaching a plate to the vertebrae of the spine, itshould be appreciated that the plates can be adopted for specificationto other parts of the body. See, for example, application Ser. No.09/022,344, filed Feb. 11, 1998, and titled Skeletal Plating System, nowU.S. Pat. No. 6,139,550, incorporated by reference above. However, thedimensions of the plate, the specific contours and placement of the bonescrew receiving holes would have to be modified.

Similarly, the bone screws described in this application could be usedin other parts of the body, again being modified so as to serve theirintended purposed, depending on the size of the body part in which theyare to be installed.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of this invention.

While specific innovative features may have been presented in referenceto specific examples, they are just examples, and it should beunderstood that various combinations of these innovative features beyondthose specifically shown are taught such that they may now be easilyalternatively combined and are hereby anticipated and claimed.

1. A plating system, comprising: an anterior cervical plate adapted tobe applied to the anterior human cervical spine, said plate having alower surface adapted to contact the anterior aspect of at least onecervical vertebral body and an upper surface opposite said lowersurface, at least one bone screw receiving hole extending from saidupper surface through said lower surface, said bone screw receiving holebeing adapted to receive at least one bone screw for engaging thecervical vertebral body to attach said plate to the cervical spine; anda bone screw adapted to attach said plate to the cervical vertebralbody, said bone screw comprising: a leading end for insertion into thecervical vertebral body, a trailing end opposite said leading end, and amid-longitudinal axis; a head adapted to block further forward motion ofsaid screw through said bone screw receiving hole of said plate, saidhead being configured not to substantially protrude beyond said bonescrew receiving hole when installed in said plate, said head having anupper portion and a lower portion, each of said upper and lower portionshaving an upper facing surface oriented at least in part toward saidtrailing end of said screw, said upper portion being configured tocooperatively engage a driver instrument adapted to insert said screwinto the vertebral body, said upper and lower portions of said head eachhaving a cross sectional dimension transverse to the mid-longitudinalaxis of said screw, said cross sectional dimension of said upper portionbeing less than said cross sectional dimension of said lower portion,said upper facing surface of said lower portion of said head being atleast in part in a plane that crosses the mid-longitudinal axis of saidscrew, said lower portion of said head having a bottom surface orientedtoward said leading end of said screw that contacts said plate; a tip atsaid leading end of said screw; a shaft between said tip and said head,said shaft having a root diameter, said shaft having a first shaftportion proximate said tip and a second shaft portion proximate saidhead, the root diameter of said first shaft portion being less than theroot diameter of said second shaft portion; and a thread along at leasta portion of said shaft, said thread adapted to engage the bone of thecervical vertebral body.
 2. The plating system of claim 1, wherein saidbottom surface of said lower portion is at least in part flat andgenerally transverse the mid-longitudinal axis of said screw.
 3. Theplating system of claim 1, wherein said upper and lower portions of saidhead each have a length parallel to the mid-longitudinal axis of saidscrew, the length of said upper portion being greater than the length ofsaid lower portion.
 4. The plating system of claim 1, wherein said upperand lower portions of said head each have a length parallel to themid-longitudinal axis of said screw, the length of said upper portionbeing less than the length of said lower portion.
 5. The plating systemof claim 1, wherein said head and said shaft each have a length parallelto the mid-longitudinal axis of said screw, the length of said headbeing less than one-half the length of said shaft.
 6. The plating systemof claim 1, wherein said head is configured to form an interference fitwith said bone screw receiving hole of said plate.
 7. The plating systemof claim 1, wherein said head has a cross sectional profile along themid-longitudinal axis that is stepped.
 8. The plating system of claim 1,wherein said cross sectional dimension of said lower portion of saidhead is configured to prevent said screw from passing completely throughsaid bone screw receiving hole of said plate.
 9. The plating system ofclaim 1, wherein said shaft has a maximum root diameter and said crosssectional dimension of said lower portion of said head is greater thanthe maximum root diameter of said shaft.
 10. The plating system of claim1, wherein said shaft has a maximum root diameter and said crosssectional dimension of said upper portion of said head is less than themaximum root diameter of said shaft.
 11. The plating system of claim 1,wherein said head comprises a recess configured to cooperatively engagea tool for installing said screw.
 12. The plating system of claim 11,wherein said recess is hex-shaped.
 13. The plating system of claim 1,wherein said upper portion of said head is non-threaded.
 14. The platingsystem of claim 1, wherein said head is substantially solid.
 15. Theplating system of claim 1, wherein at least a portion of said head issolid in a plane transverse to the mid-longitudinal axis of said screw.16. The plating system of claim 1, wherein said head is configured tocooperatively engage a locking element for retaining said screw to saidplate.
 17. The plating system of claim 1, wherein said upper facingsurface of said lower portion of said head is configured tocooperatively engage a lockingelement for retaining said screw to saidplate.
 18. The plating system of claim 17, wherein said upper portionhas a side surface generally parallel to the mid-longitudinal axis ofsaid screw that is configured to cooperatively engage the lockingelement.
 19. The plating system of claim 1, wherein said root diameterof said shaft is curved along at least a portion of the length of saidshaft in a direction between said head and said tip along themid-longitudinal axis of said screw.
 20. The plating system of claim 1,wherein said root diameter of said shaft is at least a portion of aconcave curve.
 21. The plating system of claim 1, wherein said rootdiameter has a greater rate of increase proximate said head of saidscrew.
 22. The plating system of claim 1, wherein said second shaftportion has a generally circular cross section.
 23. The plating systemof claim 1, wherein said second shaft portion is generally conical. 24.The plating system of claim 1, wherein said first shaft portion has agenerally circular cross section.
 25. The plating system of claim 1,wherein said first shaft portion is generally cylindrical.
 26. Theplating system of claim 1, wherein said tip is at least one of pointed,tapered, and coned.
 27. The plating system of claim 1, wherein said tipis configured to be self-tapping.
 28. The plating system of claim 27,wherein said tip includes at least one of a pointed tip, cutting flutes,and decreased thread height.
 29. The plating system of claim 1, whereinsaid tip includes cutting flutes that interrupt at least one turn ofsaid thread proximate said tip.
 30. The plating system of claim 1,wherein said thread has an outer diameter that diminishes proximate saidtip.
 31. The plating system of claim 1, wherein said thread has amaximum outer diameter in the range of 3.6 mm to 5.2 mm.
 32. The platingsystem of claim 1, wherein said thread pitch is in the range of 1.25 to2.5 mm.
 33. The plating system of claim 1, wherein said thread hasopposed side faces being angled relative to each other to form an apexof said thread, said side faces forming an included angle in the rangeof 11 degrees to 30 degrees.
 34. The plating system of claim 1, whereinsaid thread has opposed side faces, said side faces being angledrelative to each other to form a base at said root diameter of saidshaft and a crest opposite said base, said side faces having a thicknesstherebetween in the range of 0.25 mm to 0.60 mm at said base.
 35. Theplating system of claim 1, wherein said screw has an overall length inthe range of 10 mm to 22 mm.
 36. The plating system of claim 1, whereinsaid head has a length parallel to the mid-longitudinal axis of saidscrew in the range of 1 mm to 3 mm.
 37. The plating system of claim 1,wherein said head has a length parallel the mid-longitudinal axis ofsaid screw in the range of 2 mm and 2.5 mm.
 38. The plating system ofclaim 1, wherein at least a portion of said plating system comprises atleast in part of one of bone and bone growth promoting material.
 39. Theplating system of claim 38, wherein said bone growth promoting materialis selected from one of bone, bone derived products, bone morphogeneticprotein, and hydroxyapatite.
 40. The plating system of claim 1, incombination with a bone growth promoting material.
 41. The platingsystem of claim 40, wherein said bone growth promoting material isselected from one of bone, bone derived products, bone morphogeneticprotein, and hydroxyapatite.
 42. The plating system of claim 1, whereinat least a portion of said plating system is treated with a bone growthpromoting substance.
 43. The plating system of claim 1, wherein at leasta portion of said plating system is at least in part resorbable.
 44. Theplating system of claim 1, wherein at least a portion of said platingsystem is formed of a porous material.
 45. The plating system of claim1, wherein at least a portion of said plating system is treated topromote bone ingrowth between said plate and the adjacent vertebralbodies.